Retinoids are essential for maintenance of normal growth, development, immunity, reproduction, vision and other physiological processes. Conversely, abnormal production or processing of retinoids correlates with the manifestation of disease processes.
For example, more than 100 million of the world's children are vitamin-A deficient, causing blindness and death among these children. Excess vitamin-A levels in target organs and tissues, such as the eye, may also cause a variety of retinal diseases, including macular degeneration. A large variety of conditions, generally referred to as vitreoretinal diseases, can affect the vitreous and retina that lie on the back part of the eye, including the retinopathies and macular degenerations and dystrophies. Macular degeneration is a group of eye diseases that is the leading cause of blindness for those aged 55 and older in the United States, affecting more than 10 million Americans. Some studies predict a six-fold increase in the number of new cases of macular degeneration over the next decade, taking on the characteristics of an epidemic. Age-related macular degeneration or dystrophy, a particularly debilitating disease, leads to gradual loss of vision and eventually severe damage to the central vision.
There are two general categories of age-related macular degeneration: the wet and dry forms. Dry macular degeneration, which accounts for about 90 percent of all cases, is also known as atrophic, nonexudative, or drusenoid macular degeneration. With dry macular degeneration, drusen typically accumulates beneath the RPE tissue in the retina. Vision loss can then occur when drusen interfere with the function of photoreceptors in the macula. This form of macular degeneration results in the gradual loss of vision over many years.
Wet macular degeneration, which accounts for about 10 percent of cases, is also known as choroidal neovascularization, subretinal neovascularization, exudative, or disciform degeneration. In wet macular degeneration, abnormal blood vessel growth can form beneath the macula; these vessels can leak blood and fluid into the macula and damage photoreceptor cells. Studies have shown that the dry form of macular degeneration can lead to the wet form of macular degeneration. The wet form of macular degeneration can progress rapidly and cause severe damage to central vision.